How to set time at a specific time each day

Question

I am trying to set the system clock at a specific time each day (1:49 AM local time) so that I can send a command to close a relay at exactly 2:00 AM local time to reset the clock on a external device that can't access NTP servers. When I try to edit the crontab file, it will not let me save changes. I believe the code that I want to add is:

49 1 * * * ntpd -s 3.us.pool.ntp.org 2.us.pool.ntp.org

Can someone verify this for me? How do I edit the crontab file and save changes?

Answer 1

Just login as root and run crontab -e. A editor will appear, letting you add the cronjob.

The syntax for the cronjob is correct, running at 1:49 AM.

Crontab syntax overview (source):

# * * * * *  command to execute
# ┬ ┬ ┬ ┬ ┬
# │ │ │ │ │
# │ │ │ │ │
# │ │ │ │ └───── day of week (0 - 7) (0 to 6 are Sunday to Saturday, or use names; 7 is Sunday, the same as 0)
# │ │ │ └────────── month (1 - 12)
# │ │ └─────────────── day of month (1 - 31)
# │ └──────────────────── hour (0 - 23)
# └───────────────────────── min (0 - 59)

Answer 2

why do you need to set the system clock exactly at 2AM ? install "ntpdate" package and forget about clock syncing, it will be done totally automatically.

the problem you need to solve is to send a command to your devices to set their clocks, and that command would be, as you wrote, something along the lines of:

49 1 * * * /your/command/to/control_the_remote_device

you may enter this command in "crontab -e", which will install this as a cron job for your user (no need for root). if you don't like "vi", use "export EDITOR=nano" to set your favourite editor to be used when editing the cron file.

ps. just don't touch ntp, it works perfectly as it is =)